The present application relates generally to electronic control systems and control methods for operating one or more machines. More specifically, it relates to electronic control systems and control methods for operating one or more oil free compressors. Most specifically, it relates to electronic control systems and control methods for controlling one or more oil free two stage screw compressors.
Rotary screw compressors, such as the compressor disclosed in U.S. Pat. No. 4,435,119, have long been used to provide compressed air in industry. Such rotary screw compressor typically comprises two rotors mounted in a working space limited by two end walls and a barrel wall extending there between. The barrel wall takes the shape of two intersecting cylinders, each housing one of the rotors. Each rotor is provided with helically extending lobes and grooves that are intermeshed to establish chevron shaped compression chambers. In these chambers, a gaseous fluid is displaced and compressed from an inlet channel to an outlet channel by way of the screw compressor. Each compression chamber during a filling phase communicates with the inlet, during a compression phase undergoes a continued reduction in volume, and during a discharge phase communicates with an outlet.
Rotary screw compressors of this kind are designed to control a single stage oil flooded rotary screw compressor. The oil in the compressor does several things. First, it provides lubrication to prevent the moving parts from making contact and wearing. Second, it acts as a sealing agent to fill in all of the possible leak paths for the compressed air to escape through. Thirdly, it acts as a thermal transfer medium to absorb some of the heat of compression. The oil is discharged from the compressor with the compressed air into an oil separator tank where the oil is removed from the air. Although there is still some oil remaining in the compressed air, it is only at a level of parts per million.
It is known that these compressors may be controlled by electronic circuits, such as those disclosed in U.S. Pat. Nos. 4,336,001 and 4,227,862 to Andrew et al., which show electronically controlled startup and shutdown routines and control of a bypass slide valve to vary compressor output to maintain pressure at a selected setpoint.
U.S. Pat. Nos. 4,519,748, 4,516,914, and 4,548,549 to Murphy et al. and U.S. Pat. No. 4,609,329 to Pillis et al. show additional electronic control systems for compressors. However, the operating modalities of these systems are primarily designed for refrigerant compression.
U.S. Pat. No. 4,502,842 to Currier et al., assigned to Colt Industries Operating Corp., discloses a single electronic control system which can be connected to control a plurality of variably sized compressors. The system gathers data on the operating characteristics of the controlled compressors during a calibration phase and then uses this information to load and unload the compressors during operation, maintaining a preset pressure which can be programmed to vary with time. High and low pressure set points are programmed into the electronic control system and the compressors are selective loaded and unloaded in a predetermined sequence. However, centralized master controllers of this type represent a single point of failure for the entire pressurized air system, and are lacking in versatility since they provide only a limited selection of control modalities.
U.S. Pat. No. 4,335,582 to Shaw et al. shows a system for unloading a helical screw compressor in a refrigeration system. A slide valve is connected so that upon compressor shutdown, the slide valve is automatically driven to a full unload position. This operation is accomplished with air pressure rather than with an electronic control system.
Recently issued commonly owned U.S. Pat. No. 5,713,724 to Centers et al., the disclosure of which is herein incorporated by reference, solved a significant number of the control problems for such single stage oil flooded rotary screw compressors by providing a complete and versatile solution to the control and maintenance problems experienced when operating one or more compressors in a variety of facility installations with a variety of air storage capacities.
Oil flooded screw compressor technology has been used with great success for many years. However, the need for an oil free version of this technology is becoming more and more prevalent. Oil free compressors can provide clean air that, in most cases, requires only that any moisture content therein be removed in order to use the compressed air in many sensitive applications. Since the EPA has been diligently working to rid all manufacturing processes of any type of contamination in the environment, the fact that oil free compressors can provide air without contaminating oil. As is known, some level (at least deminimus) of oil is present in the compressed air produced by all known oil flooded screw compressors. However, an oil free compressor produces compressed air without even deminimus oil therein.
As is also known, oil free screw compressors by their very nature are complicated machines. Because of the lack of lubricant in the compressor compression chamber, timing gears are used at the ends of the rotors to prevent the rotors from rubbing together in oil free compressors. To seal the small clearances that remain after machining the compressor, all of the internal parts in the compression chamber must be coated with a material that can be worn in and also act as a lubricant in some locations inside the compression chamber. Because there is no oil in the compression chamber of an oil free compressor, there is no oil to absorb some of the heat of compression, as in oil flooded compressors. The absence of the oil or other heat absorbing material makes the oil free compressor very susceptible to rapid, uncontrolled internal temperature increase.
Further, if the oil free compressor is a two-stage compressor, the compressor control must simultaneously control both stages. Controlling a two stage compressor is very similar to controlling two separate single stage compressors. Controlling an oil free, two stage compressor or a network of oil free, two stage compressors requires a much more complex control regime than the single compressor control or a control for a network of single stage compressors, as disclosed in the aforementioned '724 patent. Each stage of oil free, two stage compressor is unloaded different from other two stage compressor design. The reason for unloading both stages is to achieve the lowest unloaded horsepower possible. By unloading both stages instead of just the first stage, unless the control regime is sufficiently advanced to detect or predict a failure condition and shut the compressor down before a compressor failure occurs, the risk of a compressor failure resulting to significant compressor damage is greatly increased.
For example, there are a number of failure modes/conditions that could result in severe compressor damage if not detected or predicated in a timely manner. One such condition is if one of the unloader valves were to fail to operate due to a condition, such as, for example, an electrical or mechanical failure. Another such condition is if one of the blowdown valves failed to operate due to an electrical or mechanical failure and caused the compressor to fail. Still another compressor failure mode would result if a coolant system failure occurred. Yet another compressor failure mode would result if the pressure of the lubricating oil used to lubricate the bearings and gears in and oil free compressor fell below a minimum pressure. Another compressor failure mode would result if the interstage pressure between the two compressor stages fell outside the normal operating parameters for the compressor. Because the interstage pressure changes, depending on whether the compressors are in a loaded or an unloaded state, a control has to determine, based on the state of the compressors, whether the interstage pressure is acceptable to continue operation or that the compressors must be shut down to avoid damaging the compressors.
Thus, there is a need for electronic control systems and control methods for operating/controlling the one or more oil free two stage compressor(s). Such systems and methods should control both stages of a oil free, two stage compressor. Such systems and methods should provide for the timely detection and/or predication of failure modes/conditions that could result in sever compressor damage. Such systems and methods should provide for the timely detection and/or predication of the failure of one of the unloader valves to fail to operate due to a condition, such as, for example, an electrical or mechanical failure. Such systems and methods should provide for the timely detection and/or predication of failure of one of the blowdown valves to operate due to an electrical or mechanical failure. Such systems and methods should provide for the timely detection and/or predication of the failure of a coolant system. Such systems and methods should provide for the timely detection and/or predication of failure of the lubricating oil to lubricate the bearings and gears in an oil free compressor and the pressure fell below a minimum pressure. Such systems and methods should provide for the timely detection and/or predication of the interstage pressure between the two compressor stages falling outside the normal operating parameters for the compressor.